This invention relates to an exposure apparatus and, more particularly, to an exposure apparatus used when a semiconductor device or the like is manufactured in a photolithography process.
A demagnifying projection exposure apparatus is used widely in the fabrication of semiconductor circuits such as LSI chips. A demagnifying projection exposure apparatus forms a reduced image of a mask pattern on a wafer, which serves as a photosensitive substrate, via projection optics. The patterns of integrated circuits projected upon a semiconductor substrate to expose the same have become increasingly finer in recent years, and there is growing demand for a projection exposure apparatus having higher resolution and alignment precision.
Improved performance of equipment peripheral to the exposure apparatus is sought in order to achieve a reduction in pattern line width. Since this results in higher equipment cost, an important challenge is to reduce the initial cost and running cost of production facilities.
In order to raise the resolution of a projection exposure apparatus, it is necessary to enlarge the numerical aperture (NA) of the projection optics and to shorten the exposure wavelength thereof. However, making the NA of the projection optics greater than a certain value is difficult in terms of the structure of the optical system. Further, when the NA of the projection optics is enlarged, the utilizable focal depth decreases and, as a result, it is difficult to realize a resolution that is possible in theory. For this reason, it is especially required that the wavelength of the exposing light be reduced in order to raise the resolution of the projection exposure apparatus.
A KrF excimer laser having a wavelength of 248 nm and an ArF excimer laser having a wavelength of 193 nm have been proposed and put into practice as exposure light sources. There is also demand for a light source having a shorter wavelength on the order of 180 nm or less, especially an F2 laser of wavelength 157.6 nm. When the wavelength of exposing light is reduced, however, absorption of the light by the optical components increases and hence, there is a limitation upon the types of glass materials that can be used as the optical components. For example, CaF2 crystal (fluorite) alone is known as a practical refraction optical material that can be used for dealing with short-wavelength light. As a consequence, it is difficult to fabricate an optical system in which various types of aberration are limited to desired values through use solely of a refraction optical system employed heretofore in projection optics.
Further, a laser light source of wavelength 200 nm or less has a certain width even if the region of oscillation wavelength is narrowed. In order to maintain good contrast and project a mask pattern, therefore, it is required that chromatic aberration be reduced to the pm (picometer) order.
A reflection optical system utilizing a concave reflecting mirror generally is used as an optical system that reduces chromatic aberration. Further, a reflection and refraction-type optical system comprising a combination of a reflection optical system and a refraction optical system is capable of reducing various aberration inclusive, especially of chromatic aberration, without inviting an increase in the number of lenses. A demagnifying projection optical system of reflection-diffraction type has, therefore, been proposed in order to eliminate chromatic aberration produced by the range of wavelengths possessed by laser light.
[Projection Optics of Reflection-refraction-type Optical System]
A projection optical system of the type disclosed in the specification of Japanese Patent Application Laid-Open No. 8-334695, for example, is a so-called off-axis optical system in which an area offset from the optical axis is used as the optical path, the off-axis arrangement affords better image quality because there is less of a decline in quality of light and no shielding of image-forming light flux. It is also easier to fabricate the various optical members.
A similar example is a projection optical system disclosed in the specification of Japanese Patent Application Laid-Open No. 2000-195772. This optical system position a concave reflecting lens in such a manner that chromatic aberration can be corrected for in excellent fashion.
Furthermore, the specification of Japanese Patent Application Laid-Open No. 2001-27727 describes a reflection-refraction-type optical system as a projection optical system using reflection-refraction-type optics. This proposed optical system has a first image-formation optical system of reflection-refraction-type for forming an intermediate image of a first surface, and a second image-formation optical system of refraction type for forming the final image of the first surface on a second surface, telecentrically based upon light from the intermediate image. This is an optical system referred to generally as a single-barrel system. Since this projection optical system has the construction of a reflection-refraction-type optical system, the projection area of a mask pattern formed on the wafer is formed at a position off-center with respect to the center of the projection optics. Physically speaking, the center of the projection optics is the center of the optical components used in the refraction optics or the center of the optical axis of the optical system. It is the optical axis AX indicated in the specification of Japanese Patent Application Laid-Open No. 2001-27727. Further, since an intermediate image resides in the projection optics and the number of times of the reflection is an even number, the visual-field area on the side of the mask also is formed at a position that is off-center on the same side as the pattern projection area mentioned above.
Furthermore, the specification of Japanese Patent Application Laid-Open No. 2001-15405 describes a reflection-refraction-type optical system, which is referred to as a twin-barrel system, as another example of a projection optical system that relies upon reflection-refraction-type optics. This system has the usually disposed projection-optics lens barrel and an additional lens barrel of the same size placed alongside. Both lens barrels are connected by yet another lens barrel. In comparison with the disclosure of Japanese Patent Application Laid-Open No. 2001-27727, therefore, the apparatus is larger in size.
[Improvement in Alignment Precision]
With regard to achieving an improvement alignment precision, the following schemes are available for dealing with the alignment of the wafer and reticle in an exposure apparatus:
1. a TTL scheme for measuring the position of an alignment mark on the wafer via the projection optics;
2. an off-axis scheme for measuring the position of an alignment mark on the wafer directly without the intervention of the projection optics; and
3. a TTR scheme for observing the wafer and the reticle via the projection optics and detecting the relative positional relationship between the two.
An example of the TTL scheme is a method of detecting an alignment mark on the wafer using light having an alignment wavelength of non-exposing light via a projection optical system referred to as TTL-AA (Though The Lens Auto Alignment). The TTL-AA scheme is advantageous for the following reason: The amount by which the wafer stage is driven at the time of alignment measurement and at the time of exposure is small because the length of line (a so-called baseline) connecting the optical axis of the projection optics and the TTL-AA optical axis is made very short. This makes it possible to suppress measurement error that occurs owing to a change in the distance between the optical axis of the projection optics and the optical axis of the TTL-AA system caused by a change in the environment surrounding the wafer stage. In other words, the advantage is small fluctuation of the baseline.
However, when the exposing light is made short-wavelength light of which the light source is an ArF or F2 laser, the glass material that can be used is limited. Consequently, correction for chromatic aberration with respect to the alignment wavelength of the projection optics is difficult. Accordingly, an off-axis scheme [referred to as xe2x80x9cOff-axis Auto-alignment (OA) detection schemexe2x80x9d below] that is not susceptible to the effects of chromatic aberration in the projection optical system is important.
In OA detection, a projection optical system does not intervene. This is advantageous in that a light source of any wavelength or a light source having a broad wavelength region can be used. One example of an advantage of using a light source having a broad wavelength region is that the effects of thin-film interference can be removed from resist with which the wafer is coated. Accordingly, it can be said that the OA detection scheme, which is capable of correcting for aberration, is an important alignment scheme with regard to light having a broad wavelength region.
In a case where alignment of the reticle and wafer is carried out using the OA detection scheme in which the relationship between an examined position and an actual exposure position is invisible, a so-called baseline quantity is obtained in advance. This is the spacing between the center of measurement of the alignment detection system and the center of the projected image of the pattern on the reticle (the center of exposure). The amount of offset from the measurement center of the alignment mark on the wafer is detected by an alignment system of the OA detection system and the wafer is moved a distance obtained by correcting the amount of offset by the baseline amount, whereby the center of the shot area is positioned accurately at the center of exposure. In the process of using the exposure apparatus, however, there are instances where the baseline quantity fluctuates gradually owing to aging. If such a change in baseline occurs, the alignment precision (overlay precision) declines. Accordingly, it is necessary to perform periodically a baseline check to accurately measure the spacing between the center of measurement of the alignment sensor and the center of exposure.
FIG. 5 is a diagram schematically illustrating the principle of baseline measurement of a projection exposure apparatus. As shown in FIG. 5, a reticle R is provided with marks RMa and RMb at positions symmetrical with respect to a center C. The reticle R is held on a reticle stage 6, which is moved in such a manner that the center C of the reticle R will coincide with the optical axis AX of projection optics 7. A wafer stage 10 is provided with a reference member FP at a position that will not interfere with a photosensitive substrate 8. The reference member FP has a reference mark FM equivalent to an alignment mark formed on the surface of the photosensitive substrate 8. If the wafer stage 10 is positioned in such a manner that the reference mark FM will arrive at a predetermined position within the projection field of view of the projection optics 7, the mark RMa of the reticle R and the reference mark FM will be detected simultaneously by a TTL-type mask alignment system 50a provided above the reticle R. If the wafer stage 10 is moved to another position, the mark RMb of the reticle R and the reference mark FM can be detected simultaneously by a mask alignment system 50b. 
The alignment sensor 16 of the OA detection system is fixedly provided exterior to the projection optics 7 (outside the projection field of view). The optical axis of the alignment sensor 16 is parallel to the optical axis AX of the projection optics 7 at the side of the projected image. A collimation mark serving as a reference when aligning the mark or the reference mark FM on the photosensitive substrate 8 is provided on a glass plate within the alignment sensor 16 and is disposed substantially in conjugation with the projection image surface (the surface of the photosensitive substrate or the surface of the reference mark FM). A laser interferometer measures the position X1 of the wafer stage 10 when the mark RMa of the reticle R and the reference mark FM on the reference member FP have been aligned using the mask alignment system 50a. Similarly, laser interferometers measure the position X2 of the wafer stage 10 when the mark RMb of the reticle R and the reference mark FM on the reference member FP have been aligned, and the position X4 of the wafer stage 10 when an index mark of the alignment sensor 16 and the reference mark FM on the reference member FP have been aligned by the mask alignment system 50b. Let X3 represent the center position of the positions X1 and X2. The position X3 resides on the optical axis AX of the projection optics 7 and is a position conjugate with the center C of the reticle.
A baseline quantity BL is obtained by calculating the difference X3xe2x88x92X4. The baseline quantity BL is a reference quantity for when the alignment mark on the photosensitive substrate 8 is subsequently aligned by the alignment sensor 16 and is fed to a point directly beneath the projection optics 7. That is, let XP represent the spacing between the center of one shot (the area to be exposed) on the photosensitive substrate 8 and the alignment mark on the photosensitive substrate 8, and let X5 represent the position of the wafer stage 10 when the alignment mark on the photosensitive substrate 8 has been aligned with the index mark of the alignment sensor 16. In order to achieve agreement between the center of the shot and the center C of the reticle, it will suffice to move the wafer stage 10 to a position expressed by the following:
(X5xe2x88x92BLxe2x88x92XP) or (X5xe2x88x92BL+XP)
Thus, by merely sensing the position of the alignment mark on the photosensitive substrate 8 using the alignment sensor 16 of the OA detection system and then feeding in the wafer stage 10 by a fixed amount that is related to the baseline quantity BL, the pattern on the reticle R can immediately be overlaid accurate on the shot area of the photosensitive substrate 8 to carry out exposure. It should be noted that although only a single dimension has been considered here, in actuality it is required to take two dimensions into consideration.
An apparatus described in the specification of Japanese Patent Application Laid-Open No. 9-219354 has been proposed as an example of the OA detection system in the prior art. The apparatus described in Japanese Patent Application Laid-Open No. 9-219354, which takes into account a short-term change in the measurement center of the alignment sensor, provides an index mark on the objective lens of the alignment sensor and, to the extent possible, constructs the detection system for the index mark and the detection system for the mark on the wafer of common elements, thereby reducing the effects of drift caused by heat or mechanical vibration applied to the detection systems. This improves detection precision.
When the above-described OA detection system is thus used at a position detection position for detecting the position of the wafer, the wafer stage must be driven at the time of alignment measurement and at the time of exposure because the optical axis of the projection optics and the detection area of the OA detection system are spaced apart. Owing to a change in the environment in the vicinity of the wafer stage, therefore, alignment precision is affected by fluctuation of the baseline. For example, the influence of fluctuation of the air varies depending upon a difference in interferometer length, thereby giving rise to interferometer measurement error. This causes a difference between the wafer drive grid at the time of exposure and the wafer drive grid at the time of alignment detection, thereby giving rise to a decline in alignment precision.
Accordingly, shortening the baseline as much as possible and making the environment of the wafer stage the same at the time of alignment and at the time of exposure is effective in eliminating such a decline in alignment precision. To achieve this, it is necessary that the OA detection system be placed at such a position.
The specification of Japanese Patent Application Laid-Open No. 2000-91219 describes an OA detection system in which an objective lens includes, in the order mentioned starting from the wafer side, a first lens group having a positive refracting power, a mirror for reflectively deflecting luminous flux from the first lens group, and a second lens group for condensing the luminous flux from the mirror, the system having an image sensing element for opto-electronically converting an alignment-mark image formed by the objective lens.
Another example proposed in the specification of Japanese Patent Application Laid-Open No. 11-345761 is to dispose an OA detection system in an EUV (Extreme Ultraviolet) projection apparatus in which EUV light having a wavelength of 5 to 20 nm is used as the exposing light. In accordance with this proposal, the projection optical system is constituted by a reflection optical system, in which part of a reflecting mirror constructed on the wafer side and through which the exposing light passes is cut away to provide a space. The baseline is made as short as possible by placing the OA detection system in this space.
[Reducing Apparatus Size]
Reducing the size of the exposure apparatus and peripheral equipment is vital in terms of reducing the cost of the semiconductor production facilities. The initial cost and running cost can be reduced by reducing the size of the clean room in which the exposure apparatus and peripherals are placed. Alternatively, a greater number of exposure apparatus and peripherals can be installed within a clean room to increase production volume. A reduction in the size of the exposure apparatus is proposed in the specification of Japanese Patent Application Laid-Open No. 4-352608. The exposure apparatus disclosed in Japanese Patent Application Laid-Open No. 4-352608 is reduced in size by placing an air conditioner filter in what was dead space within the chamber of the conventional exposure apparatus, whereby space is utilized more efficiently.
A cause of the baseline fluctuation that is a problem in OA detection systems is not merely drift of the measurement center position in the alignment sensor. Other examples are fluctuation in the position of the overall alignment sensor relative to the center of the projected image, and the precision with which the wafer stage is moved. Let BL represent distance from the center of the projected image to the measurement center of the alignment sensor. A change in temperature of one degree centigrade gives rise to thermal deformation of the structure supporting the projection optics and alignment sensor, where the deformation is equal to the product of BL and the coefficient of thermal expansion of the structure. If there is a yawing component xcex8, for example, a measurement equal to xcex8xc3x97BL will arise with regard to the precision with which the wafer stage is moved. Such deformation or error constitutes alignment error. In particular, since the alignment sensor of the OA detection system does not rely upon intervention of projection optics when the alignment mark on the wafer is detected, it is important that the influence of measurement error due to drift be minimized as compared with a TTL-based alignment sensor that relies upon intervention of projection optics.
In a projection exposure apparatus that uses a KrF or ArF excimer laser beam or an F2 laser beam as illuminating light for the purpose of exposure, the adoption of a TTL-type alignment sensor is accompanied by a variety of technical difficulties. The off-axis alignment sensor having a high degree of freedom of design and potential functionality, therefore, takes on increased importance. However, in a case where alignment is performed using the off-axis scheme, the precision of alignment will decline in comparison with alignment using the TTL scheme unless the effects of measurement error due to baseline fluctuation are mitigated, as mentioned above.
As set forth above, reducing the wavelength of the exposing light is effective in raising the resolution of the exposure apparatus, and adopting a reflection or a reflection-refraction optical system in order to achieve this reduction in wavelength. In addition, shortening the baseline is effective in improving alignment precision. Further, reducing the size of the exposure apparatus is effective from the standpoint of lowering the cost of the semiconductor manufacturing facilities. There is no prior-art system that offers all three of these features, namely, use of a reflection-refraction optical system as the projection optical system, shortening of the baseline and reduction in the size of the apparatus.
For example, the so-called single-barrel optical projection system disclosed in the specification of Japanese Patent Application Laid-Open No. 2001-27727 having a reflection-refraction optical system as the projection optical system succeeds in raising resolution but not in improving alignment precision. Though it is possible to shorten the baseline by building the OA detection system of Japanese Patent Application Laid-Open No. 2000-91219 into the exposure apparatus having the single-barrel optical projection system disclosed in Japanese Patent Application Laid-Open No. 2001-27727, the size of the apparatus is likely to be increased by an amount equivalent to the offset of the pattern projection area from the center of the projection optical system. A wafer stage that holds and moves a wafer is required to have a movable area that allows the entire wafer surface to be exposed, with the pattern projection area being at the center. If the pattern projection area deviates from the center of the projection optical system, it is required that the placement of the wafer stage be shifted with respect to the projection optical system by the amount of deviation. The problem is that the size of the apparatus increases by an amount commensurate with the amount of the shift.
The so-called twin-barrel projection optical system disclosed in the specification of Japanese Patent Application Laid-Open No. 2001-15405 succeeds in raising resolution, but not in improving alignment precision. Even if the OA detection system disclosed in the specification of Japanese Patent Application Laid-Open No. 2000-91219 is incorporated, as in the manner of the above-described single-barrel scheme, the very fact that the arrangement has twin barrels results in an apparatus of a large size.
Further, the EUV exposure apparatus disclosed in the specification of Japanese Patent Application Laid open No. 11-345761 succeeds in raising the resolution of the exposure apparatus and in improving alignment precision, but has a projection optical system that itself is large in size. This results in a large-size apparatus. With regard to the improvement in alignment precision, the fact that the projection optical system is of a reflection type means that the optical components constructed on the side of the wafer are mirrors. This system is realized by cutting away the portion through which the exposing light is transmitted. If the single-barrel reflection-refraction optical system is adopted, using mirrors for the optical components arranged on the side of the wafer poses difficulties in terms of optical design. As a consequence, the apparatus is limited to a case where the reflection-type optical system is used.
Furthermore, since this apparatus is an EUV exposure apparatus, the arrangement is premised on the fact that a vacuum exists within the apparatus. A chamber having a vacuum-supporting structure is required and, as a result, there is a further increase in the size of the apparatus. Even if a vacuum pump for establishing the vacuum is constructed on the facility side or on the side of the exposure apparatus, the size of the apparatus is increased owing to the provision of the pump. Furthermore, there is a marked increase in power consumption and an increase in the cost of the manufacturing facilities.
In the case of Japanese Patent Application Laid-Open No. 11-345761, the mask illumination area is placed at a position that is off-centered by an amount greater than the amount by which the pattern exposure area is offset from the center of the optical projection system. Moreover, the mask illumination area is off-centered in a direction opposite from that in which the pattern projection area is formed relative to the center of the projection optical system. This increases the size of the apparatus even further.
In the case also of the single-barrel optical projection system disclosed in the specification of Japanese Patent Application Laid-Open No. 2001-27727, an offset from the center of the projection optical system results not only in off-centering of the pattern projection area but also in off-centering of the reticle illumination area in a manner similar to that of the exposure apparatus described in Japanese Patent Application Laid-Open No. 11-345761. Since the relationship between the amount of off-centering and the size of the exposure apparatus is not taken into account at all, the amount of off-centering itself leads to an increase in the size of the apparatus.
Further, the exposure apparatus disclosed in Japanese Patent Application Laid-Open No. 4-352608 is satisfactory in terms of size reduction but does not deal with improvements in resolution and alignment.
The present invention has been devised in view of the above-mentioned problems and its object is to provide an exposure apparatus in which the effects of measurement error due to baseline fluctuation can be mitigated and the position of an object to be detected (the position detection mark) can be detected highly precisely so that highly accurate alignment can be achieved.
Another object of the present invention is to provide an exposure apparatus in which resolution is raised by reducing the wavelength of exposing light through utilization of a reflection-refraction optical system, in which alignment precision is improved by shortening the baseline, and in which the apparatus can be reduced in size.
According to the present invention, the foregoing objects are attained by providing an exposure apparatus comprising: an illuminating optical system for illuminating a reticle with illuminating light from a light source; a projection optical system for projecting a pattern, which has been formed on the reticle, onto a photosensitive substrate; and a position detection system for detecting an alignment mark on the substrate; wherein a pattern projection region for projecting the pattern onto the substrate by the projection optical system is formed at a position off-centered toward the side of the position detection system from the projection center of the projection optical system.
The present invention further provides a device manufacturing method for manufacturing a device by the above-described exposure process.
The present invention further provides a semiconductor device manufacturing plant having a group of manufacturing equipment for various processes, inclusive of any of the exposure apparatus described above, wherein information relating to at least one of the pieces of manufacturing equipment can be communicated by data communication using a local-area network and/or an external network outside the plant.
The present invention further provides a method of maintaining the above-described exposure apparatus.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.